The present invention relates to a three-phase nozzle centrifuge for separating a light phase having a relatively low density, a heavy phase having a relatively high density and a solid phase from a mixture containing these three phases. The light and heavy phases may, for instance, be constituted by oil and water, resp., and the solid phase may, for instance, be constituted by sand. It is very important for a satisfactory separation that the interface layer between the light and heavy phases is placed correctly. The movement of this layer even a fraction of a centimeter in the centrifuge will substantially affect the separation results: A movement radially outward will produce drier oil and water containing some oil; a movement radially inward will produce oil with higher moisture content and more oil-free water; and a placement in a position radially as far out as possible will effect a larger amount of rupturing or breakage but without a substantially improved separation.
Generally, a centrifugal separator may have outlets for the separated liquids formed in several different ways. For example, the rotor may be provided with so-called overflow outlets for both of the liquids or an overflow outlet for one liquid and another kind of outlet for the other liquid, wherein the latter may be constituted, for instance, by a non-rotatable so-called paring disc or by nozzles situated in the surrounding wall of the rotor. Nozzles are used as a rule when the supplied mixture also contains solids which are heavier than the two liquids as in the present case. Then, separated solids together with part of the heavy liquid may be discharged through these nozzles placed at the periphery of the rotor, whereas the separated light liquid is discharged from a central part of the rotor through an overflow outlet or a paring disc. In these cases, the rotor may also form a space which communicates with the radially outer part of the separation chamber in such a way that during a separating operation it will contain separated heavy liquid but not separated light liquid. Excess separated heavy liquid which does not leave the separation chamber through said nozzles is then discharged from the rotor through this space.
In a nozzle centrifuge of the kind described above it may be difficult to maintain the interface layer which is formed in the rotor between the liquids separated therein at a predetermined radial level during a separating operation. However, keeping the radial level of the interface layer within a predetermined range is important for obtaining optimal separation results. One of the reasons is that an uncontrollable amount of separated heavy liquid leaves together with the separated solids through the so-called sludge outlet of the rotor; another reason is wear of the outlet nozzles which leads to higher outlet rates for the heavy liquid over the time. Thus, if the amount of heavy liquid leaving the rotor exceeds the amount of heavy liquid being introduced into the rotor together with the mixture to be treated therein, the interface layer in the separating chamber between light liquid and heavy liquid will move radially outwardly, with the result initially mentioned, i.e., separated light liquid being lost together with the separated heavy liquid leaving the rotor.
In particular, this applies to the cleaning of oil from sand and water in connection with recovery of oil from so-called tar sands. In this connection, nozzle centrifuges are used in several separating steps. In a first separating step, a mixture of oil, water and sand residues is introduced into a nozzle centrifuge. The sand and the main part of the supplied water leave the centrifuge rotor through its nozzles, whereas a certain amount of water is removed from the rotor through a central overflow outlet. Separated oil is conducted out of the rotor from a central part thereof, for example, through a paring disc, and is pumped further to another nozzle centrifuge to go through a second separating step. Water is added separately in excess or in a balanced manner with a VCD (“virtual capacity drum”) such that the interface layer formed in the separating chamber of the rotor between oil and water shall not be displaced radially outwardly, even after many hours' operation of the centrifugal separator when its nozzles have become worn of the outflowing sand and, therefore, let out more water than at the beginning of the separating operation as already explained above.
After the first separating step, the oil still contains residues of sand and water, however. In order to enhance the separation result, control equipment comprising a pressure vessel has been developed for controlling the separating operation in the second separating step. By means of this control equipment, it is possible to avoid continuous addition of an excess amount of water to the mixture being introduced into the centrifugal rotor. Instead, water is introduced into the separating chamber of the rotor through a space in the rotor of the kind previously described (i.e., a space communicating only with the radially outer part of the separating chamber) only when needed. Through the same space, water is also removed from the rotor when excess water enters together with the oil to be cleaned which excess water cannot leave the rotor through the sludge outlet nozzles. It should be noted that it is also possible to supply additional water already in the first step—in this case, the difference between the first and second steps is that, during the first step, compensation water is added at certain abnormal conditions, whereas, in the second step, the supply of water is continuously regulated. In any case, the control equipment which has been developed is expensive and complicated. For each one of a great number of nozzle centrifuges it comprises a pressure vessel for water, the lower part of which communicates with a liquid paring disc (which is situated in said space in the rotor of the centrifugal separator) through a conduit for the introduction of water into or discharge of water out of the rotor. In the upper part of the pressure vessel a gas pressure is maintained (usually by means of nitrogen gas) the magnitude of which is continuously controlled in response to the amount of water which is present in the pressure vessel, so that the liquid pressure at the bottom of the pressure vessel (and, thus, within the conduit through which the pressure vessel communicates with said space in the centrifugal rotor) is always kept constant at a predetermined value. The constant value of the liquid pressure in said conduit corresponds to a desired radial level of the interface layer formed in the separating chamber of the rotor between separated oil and separated water. If the interface layer moves radially outwardly from the desired level the pressure in said space in the rotor drops, with the result that water is pressed from the pressure vessel through said conduit into the rotor until the interface layer has returned to the desired radial level. A level sensor in the pressure vessel is adapted to initiate the supply of new water to the pressure vessel upon need, so that it will never be empty of water. If the interface layer in the separating chamber of the rotor starts to move radially inwardly from the desired level, the pressure in said space in the rotor increases, with the result that excess water is pressed from this space through said conduit into the pressure vessel. When the liquid level in the pressure vessel has risen to an upper limit level, a bottom outlet of the pressure vessel is opened for release of water therefrom.
WO 00/37177 discloses a control equipment of this kind which is amended in that respect that the pressure source for control liquid which is part of the supply device is not integrated in the discharge device. The separated heavy liquid and/or control liquid leaving the rotor need not be accumulated at an elevated pressure and, consequently, no pressure vessel is needed. Furthermore, there is no need for a system for compression of gas and for control of the pressure of such a gas. Instead, the pressure source may be constituted by a simple liquid pump and the whole control of the supply of controlling liquid and discharge of separated heavy liquid and/or control liquid can be performed by means of one or more so-called constant pressure valves. If a container is needed for a buffer amount of control liquid, such a container may be free of pressure and common to several centrifugal separators.
Another approach for controlling the location of the interface layer—i.e., for compensating possible loss or excess of the heavy liquid—does not make use of pressure vessels or valves for actively moving the interface layer. Instead, the radial position of the interface layer is maintained by providing the outlet for the lighter liquid phase with an overflow outlet in the shape of a so-called level ring having an opening surrounding the rotation axis and the outlet for the heavier liquid phase with an overflow outlet also in the shape of a level ring having an opening surrounding the rotational axis, wherein additional auxiliary heavier liquid is supplied, for example, through the outlet for the heavier liquid phase. Since the radial position of the interface layer is governed by the relative radial positions of the inner edges of the level rings with respect to each other, i.e., the ratio of their openings, adjustment of the interface layer could be achieved by stopping the centrifugal separator and replacing one or both level rings by a level ring or rings having another radius for the respective overflow outlet. This would have to be done several times before the right level ring or rings are found, i.e., a level ring or rings having a radius for the respective overflow outlet leading to the right position of the interface layer and, thus, to a satisfactory separation result. Obviously, this would constitute a difficult and time consuming operation.
In order to overcome this problem, U.S. Pat. No. 3,047,214 proposes to choose appropriate level rings for the centrifuge in view of the particular separation process at the beginning and then to adjust the radial position of the interface layer by adding the auxiliary heavier liquid at variable density. The interface is moved radially inwardly by increasing the density of the auxiliary heavier liquid—for example, by dissolving salt of a higher density therein—or radially outwardly by decreasing the density of the auxiliary heavier liquid—for example, by dissolving a liquid with lower density therein such as an alcohol.
Still another approach to overcome the problem of correctly positioning the interface layer is disclosed in WO 00/74858. Here, the centrifugal separator is provided with a paring device which is movable between different radii in an outlet chamber. This paring device is initially (i.e., when the separator is empty) moved to a radial inner position in the outlet chamber and then (after the mixture has entered the separator) it is moved towards the free liquid surface in the outlet chamber until the paring device reaches the liquid surface where it is prevented from moving further radially outwardly—this substantially corresponds to a wanted position of the interface layer.
All these approaches still have drawbacks, however. For example, the active approach with pressure vessels or valves results in considerable generation of erosion and heat in the discharge because the stationary discharge device is partly immersed in the rotating liquid body in the outlet chamber. The approaches with level rings or movable paring devices do not offer an acceptable control of the radial level of the interface layer because the feeding of additional heavy phase (i.e., water) is not accurately monitored. In addition thereto, the level ring approach leads to a very high consumption of auxiliary liquid and power because excess auxiliary heavier liquid which is not admitted to the separator is disposed of and, thus, wasted. Finally, the kind of separator in question also plays an important role when choosing detailed design options: The problem and type of interface control for intermittently discharging centrifuges is quite different from the problem and type of interface control for nozzle centrifuges which are designed for continuous discharge of solids as well as part of the heavy phase.